Method of Making a Translucent Colored Zirconia Dental Restoration

ABSTRACT

A method of making a translucent colored zirconia dental restoration comprises obtaining a zirconia green body, forming a dental restoration precursor from the zirconia green body, applying a color liquid to the precursor, and sintering the restoration precursor with regular sintering in air without post HIP processing. The zirconia green body comprises between 7 wt % to 20 wt % of stabilizer based on a total weight percent, and an L* value between 10 and 20 for a sample thickness of 1 to 1.3 mm. The zirconia green body is subsequently finally sinterable with regular sintering in air without post HIP processing to produce a translucent zirconia sintered body having a total light transmittance of at least 36% and less than 50% to light with a wavelength of 400 nm, and less than 55% to light with a wavelength of 600 nm, at a thickness of 0.6 mm.

PRIORITY CLAIM(S)

This is a continuation of U.S. patent application Ser. No. 16/425,424,filed May 29, 2019; which is a continuation of U.S. patent applicationSer. No. 15/945,529, filed Apr. 4, 2018; which is a continuation of U.S.patent application Ser. No. 15/906,939, filed Feb. 27, 2018; which is acontinuation of U.S. patent application Ser. No. 15/645,626, filed Jul.10, 2017; which is a continuation of U.S. patent application Ser. No.15/595,365, filed May 15, 2015, which is abandoned; which is acontinuation of U.S. patent application Ser. No. 14/559,571, filed Dec.3, 2014, now U.S. Pat. No. 9,649,179, which is a continuation of U.S.patent application Ser. No. 13/403,417, filed Feb. 23, 2012, now U.S.Pat. No. 8,936,848, which are hereby incorporated herein by reference.

RELATED APPLICATION(S)

This is related to U.S. patent application Ser. No. 13/403,494, filedFeb. 23, 2012; which is hereby incorporated herein by reference.

BACKGROUND Field of the Invention

The present invention relates generally to dental blanks for formingdental prostheses. More particularly, the present invention relates to agreen body zirconia dental blank with at chemical compositions ofincreasing amounts of yttria through a thickness thereof and apre-sintered optical characteristic of chroma that is substantiallyconsistent and white across the thickness; and being milled, colored andsintered to form the dental prosthesis with an optical characteristic ofdecreasing chroma through a thickness of the dental prosthesis aftersintering.

Related Art

There are three main classes of dental ceramics: Group I—predominantlyglassy materials; Group II—particle-filled glasses and glass-ceramics asa special subset of particle-filled glasses; and GroupIII—polycrystalline ceramics.

Group I—predominantly glassy ceramics—are 3-D networks of atoms havingno regular pattern to the spacing between nearest or next nearestneighbors, thus their structure is ‘amorphous’ or without form. Glassesin dental ceramics derive principally from a group of mined mineralscalled feldspar and are based on silica (silicon oxide) and alumina(aluminum oxide), hence feldspathic porcelains belong to a family calledalumino-silicate glasses.

Group II—particle-filled glasses and glass-ceramics—have fillerparticles that are added to the base glass composition in order toimprove mechanical properties and to control optical effects such asopalescence, color and opacity. These fillers are usually crystallinebut can also be particles of a higher melting glass. Glass-ceramics inGroup II have crystalline filler particles added mechanically to theglass, e.g. by simply mixing together crystalline and glass powdersprior to firing. In a more recent approach, the filler particles aregrown inside the glass object (prosthesis) after the object has beenformed. After forming, the glass object is given a special heattreatment, causing the precipitation and growth of crystallites withinthe glass. Such particle-filled composites are called glass-ceramics.More recently a glass-ceramic containing 70 vol % crystalline lithiumdisilicate filler has been commercialized for dental use. Example ofthis is Empress 2, now e.maxPress and e.maxCAD from IvoClar-Vivadent.

Group III—polycrystalline ceramics—have no glassy components; all of theatoms are densely packed into regular arrays that are much moredifficult to drive a crack through than atoms in the less dense andirregular network found in glasses. Hence, polycrystalline ceramics aregenerally much tougher and stronger than group I and II glassy ceramics.Polycrystalline ceramics are more difficult to process into complexshapes (e.g. a prosthesis) than are glassy ceramics and tend to berelatively opaque compared to glassy ceramics. (Ceramic materials indentistry: historical evolution and current practice (2011), JR Kelly,University of Connecticut Health Center, Department of ReconstructiveSciences, Farminton, Conn.).

Advanced polycrystalline ceramic materials such as zirconia have greatpotential as substitutes for traditional materials in many biomedicalapplications. Since the end of the 1990s, the form of partiallystabilized zirconia has been promoted as suitable for dental use due toits excellent strength and superior fracture resistance. In addition,zirconia bio-ceramic presents enhanced biocompatibility, lowradioactivity, and good aesthetic properties. The introduction ofcomputer-aided design/computer-aided manufacturing (CAD/CAM) techniqueshas increased the general acceptance of zirconia in dentistry.

Zirconium dioxide (ZrO2) known as zirconia, is a crystalline oxide ofzirconium. Although pure zirconium oxide does not occur in nature, it isfound in the minerals baddeleyite and zircon (ZrSiO4). At ordinarytemperatures, it has a hexagonal close-packed crystalline structure andforms a number of compounds such as zirconate (ZrO3-2) and zirconyl(ZrO+2) salts. Zirconia is obtained as a powder and possesses bothacidic and basic properties. Zirconium oxide crystals are arranged incrystalline cells (mesh) which can be categorized in threecrystallographic phases: 1) the cubic (C) in the form of a straightprism with square sides 2) the tetragonal (T) in the form of a straightprism with rectangular sides and 3) the monoclinic (M) in the form of adeformed prism with parallelepiped sides. The cubic phase is stableabove 2,370° C. and has moderate mechanical properties, the tetragonalphase is stable between 1,170° C. and 2,370° C. and allows a ceramicwith improved mechanical properties to be obtained, while the monoclinicphase, which is stable at room temperatures up to 1,170° C., presentsreduced mechanical performance and may contribute to a reduction in thecohesion of the ceramic particles and thus of the density.

Partially stabilized zirconia is a mixture of zirconia polymorphs,because insufficient cubic phase-forming oxide (stabilizer) has beenadded and a cubic plus metastable tetragonal ZrO2 mixture is obtained. Asmaller addition of stabilizer to the pure zirconia will bring itsstructure into a tetragonal phase at a temperature higher than 1,000° C.and a mixture of cubic phase and monoclinic (or tetragonal) phase at alower temperature. This partially stabilized zirconia is also calledtetragonal zirconia polycrystal (TZP). Several different oxides, eg,magnesium oxide (MgO), yttrium oxide, (Y₂O₃), calcium oxide (CaO), andcerium oxide (Ce2O3), can be added to zirconia to stabilize thetetragonal and/or cubic phases.

Nowadays dental restorations or prostheses are often made using zirconiaceramic with CAD (Computer Aided Design) and CAM (Computer AidedMachining) process, which typically includes:

-   -   capturing data representing the shape of a patient's teeth, for        example by scanning a plaster model of the patient's teeth or        alternatively by scanning the actual teeth in the patient's        mouth;    -   designing the shape of a dental restoration precursor based on        the captured data using software, such as computer-aided design        (CAD) software;    -   machining the dental restoration precursor to correspond to the        designed shape, for example, by an automated Computer Numerical        Controlled (CNC) machine; and    -   optionally finishing the dental restoration precursor by        sintering and/or veneering.

A common method of making dental restorations includes milling arestoration precursor out of a zirconia disc/blank of a pre-sintered butstill porous ceramic material. The disc/blank is typically formed bycompacting an amount of ceramic powder. The zirconia disc/blank ofcompacted powder is usually subsequently pre-sintered to provide it withthe required mechanical stability for handling and machining. Once therestoration precursor has been obtained from machining the disc/blankthe precursor is typically sintered in the further process of making thefinal dental restoration. During sintering the precursor typicallyshrinks, generally proportionally, because the initially porous materialreduces in porosity and increases in density. For this reason therestoration precursor may be initially larger, for example about 18 to27%, than the desired final shape after sintering, to account forshrinkage during the sintering step. To form the final dentalrestoration the sintered restoration precursor may be veneered orotherwise finished.

Some Group I or II glass ceramic blocks already have different upper andlower optical properties, such as translucency, brightness, reflectanceand color. Thus, the glass ceramic block itself already haspre-determined optical properties. For example, see U.S. Pat. No.8,025,992. Such a pre-colored glass ceramic block can be used primarilyin dentists' office with a view to finish the indirect treatment withjust one visit. The dental laboratory can also be a user. Here theindirect treatment mainly means to put a crown, bridge, inlay and/oronlay in replacement of the damaged tooth.

After the dentist preps the tooth, he/she chooses a glass ceramic blockthat already has color in it. Each layer of the block has a colorprofile already integrated into the block after the pre-sintered stageand it is implied that each layer should not be different in chemicalcharacteristics. For each layer, coloring is affected only by additionof coloring oxides to the melt from which the granulate is obtained, orto the ground granulate and not due to differing chemicalcharacteristics. These oxides are then present separately. In summary,these pre-colored, glass ceramic blocks already have predetermined,built-in optical properties in the block

It can be advantageous for small, ready-to-be-used individual blocks tohave these built-in optical characteristics for small production. Butpre-colored individual blocks can also be disadvantageous for massproduction of prostheses of various sizes and various desired opticalcharacteristics.

The milling machine mills the pre-colored glass ceramic block one at atime in a single mill sequence. The inefficiency with this ceramic blockis that if there are 15 different colors of prosthetic teeth to bemilled, then the machine should be stopped each time so that the milledblock could be removed and each different block could be loaded. Thus,the pre-colored glass ceramic blocks are not efficient for dentallaboratories where numerous cases should be milled, regardless of theoptical properties of the dental prostheses. These laboratories need toreduce the stopping of the machines as much as possible to save time andincrease productivity.

For examples of pre-colored dental blocks, see U.S. Pat. Nos. 8,025,992and 7,981,531; and US Patent Publication No. 2011-0236855. For examplesof zirconia dental blocks, see U.S. Pat. Nos. 7,011,522 and 6,354,836.

SUMMARY OF THE INVENTION

It has been recognized that it would be advantageous to develop a dentalprosthesis with improved or more natural optical characteristics, suchas translucency and/or chroma, and/or with different layers havingdifferent optical characteristics. In addition, it has been recognizedthat it would be advantageous to develop a green body dental blankhaving different layers of different chemical compositions, butsubstantially consistent optical characteristics prior to sintering, andwhich can be milled, colored and sintered to obtain layers of differentoptical characteristics.

The invention provides a method of making a translucent colored zirconiadental restoration. The method comprises the step of obtaining azirconia green body comprising zirconium oxide and between 7 wt % to 20wt of another oxide based on a total weight percent of the zirconiagreen body, and an L* value between 10 and 20 for a sample thickness of1 to 1.3 mm in accordance with CIE L*a*b* colorimetric system, measuredwith a reading tip of a spectrophotometer flush with, in close touchingcontact, and perpendicular to a measured surface of the sample. Inaddition, the zirconia green body is subsequently finally sinterablewith regular sintering in air without post HIP processing to produce atranslucent zirconia sintered body having a total light transmittance ofat least 36% and less than 50% to light with a wavelength of 400 nm, andless than 55% to light with a wavelength of 600 nm, at a thickness of0.6 mm measured using a LAMBDA 35 UV/VIS Spectrophotometer manufacturedby Perkin Elmer. The, the method comprises forming a dental restorationprecursor from the zirconia green body. Then, the method comprisesapplying a color liquid to the precursor. And then, the method comprisessintering the restoration precursor with regular sintering in airwithout post HIP processing resulting in the translucent coloredzirconia dental restoration.

In addition, the invention provides a method of making a translucentcolored zirconia dental restoration. The method comprises the step ofobtaining a zirconia green body comprising zirconium oxide and between6.5 to 20 wt % of another oxide, based on a total weight percent of thezirconia green body, and an L* value between 10 and 20 for a samplethickness of 1 to 1.3 mm in accordance with CIE L*a*b* colorimetricsystem, measured with a reading tip of a spectrophotometer flush with,in close touching contact, and perpendicular to a measured surface ofthe sample. In addition, the zirconia green body is subsequently finallysinterable with regular sintering in air without post HIP processing toproduce a translucent zirconia sintered body having a total lighttransmittance of at least 35% and less than 50% to light with awavelength of 400 nm, and at least 48% to light with a wavelength of 500nm, and less than 55% to light with a wavelength of 600 nm, at athickness of 0.6 mm, measured using a LAMBDA 35 UV/VIS Spectrophotometermanufactured by Perkin Elmer. Then, the method comprises forming adental restoration precursor from the zirconia green body, and applyinga color liquid to the precursor. And then, the method comprisessintering the restoration precursor with regular sintering in airwithout post HIP processing resulting in the translucent coloredzirconia dental restoration.

In addition, the invention provides a method of making a translucentcolored zirconia dental restoration. The method comprises the step ofobtaining a zirconia green body with an L* value between 10 and 20 for asample thickness of 1 to 1.3 mm in accordance with CIE L*a*b*colorimetric system, measured with a reading tip of a spectrophotometerflush with, in close touching contact, and perpendicular to a measuredsurface of the sample. In addition, the zirconia green body issubsequently finally sinterable with regular sintering in air withoutpost HIP processing to produce a translucent zirconia sintered bodyhaving a total light transmittance of greater than 37% and less than 50%to light with a wavelength of 400 nm, and greater than 47% to light witha wavelength of 500 nm, and less than 55% to light with a wavelength of600 nm, at a thickness of 0.6 mm, measured using a LAMBDA 35 UV/VISSpectrophotometer manufactured by Perkin Elmer. The, the methodcomprises forming a dental restoration precursor from the zirconia greenbody, and applying a color liquid to the precursor. And then, the methodcomprises sintering the restoration precursor with regular sintering inair without post HIP processing resulting in the translucent coloredzirconia dental restoration.

Furthermore, the invention provides a method of making a translucentcolored zirconia dental restoration. The method comprises the step ofobtaining a zirconia green body comprising: zirconium oxide and greaterthan 6.5 wt % and less than 20 wt % of another oxide based on a totalweight percent of the zirconia green body; an L* value between 10 and 20for a sample thickness of 1 to 1.3 mm in accordance with CIE L*a*b*colorimetric system, measured with a reading tip of a spectrophotometerflush with, in close touching contact, and perpendicular to a measuredsurface of the sample; and a chemical composition with color pigments.In addition, the zirconia green body is subsequently finally sinterableat a temperature of at least 1300° C. with regular sintering in airwithout post HIP processing to produce a translucent zirconia sinteredbody having a total light transmittance of less than 50% to light with awavelength of 400 nm, and less than 55% to light with a wavelength of600 nm, and at least 43% to light with a wavelength at a point between400 nm and 600 nm, and at least 50% to light with a wavelength at apoint between 600 nm and 800 nm, at a thickness of 0.6 mm measured usinga LAMBDA 35 UV/VIS Spectrophotometer manufactured by Perkin Elmer. The,the method comprises forming a dental restoration precursor from thezirconia green body. And then, the method comprises sintering therestoration precursor with regular sintering in air without post HIPprocessing resulting in the translucent colored zirconia dentalrestoration.

The current invention zirconia material can be used to manufacturedental prostheses including, but not limited to, crowns, partial crowns,bridges, inlays, onlays, orthodontic appliances, space maintainers,tooth replacement appliances, splints, dentures, posts, facings,veneers, facets, implants, abutments, cylinders, and connectors.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional features and advantages of the invention will be apparentfrom the detailed description which follows, taken in conjunction withthe accompanying drawings, which together illustrate, by way of example,features of the invention; and, wherein:

FIG. 1 is a flowchart showing a method of making a green body dentalblock and a dental prosthesis in accordance with an embodiment of thepresent invention;

FIG. 2a is a perspective view of the green body dental block inaccordance with an embodiment of the present invention;

FIG. 2b is a schematic perspective view of the green body dental blockof FIG. 1 shown with multiple different layers and a dental prosthesisto be milled therefrom;

FIG. 2c is a schematic perspective view of another green body dentalblock in accordance with another embodiment of the present invention;

FIG. 2d is a schematic perspective view of another green body dentalblock in accordance with another embodiment of the present invention;

FIG. 2e is a schematic perspective view of another green body dentalblock in accordance with another embodiment of the present invention;

FIG. 3a is a schematic perspective view of the green body dental blockof FIG. 1 shown with multiple different layers and a dental prosthesisand a sample disc to be milled therefrom;

FIG. 3b is a schematic view of a green body dental prosthesis milledfrom the green body dental blank of FIG. 3a shown with multipledifferent layers;

FIG. 3c is a schematic view of a green body sample disc milled from thegreen body dental blank of FIG. 3a shown with multiple different layers;

FIG. 3d is a schematic view of a portion of the green body showing theopen pores between grains thereof;

FIG. 3e is a graph of the translucency of the multiple different layersof the green body dental prosthesis of FIG. 3 b;

FIG. 3f is a graph of the chroma level of the multiple different layersof the green body dental prosthesis of FIG. 3 b;

FIG. 3g is schematic view of a dental prosthesis milled from the greenbody dental blank of FIG. 3a shown with multiple different layers;

FIG. 3h is a schematic view of a sample disc milled from the green bodydental blank of FIG. 3a shown with multiple different layers;

FIG. 3i is a graph showing translucency and chroma levels for the dentalprosthesis of FIG. 3 g;

FIG. 3j is a graph showing translucency, chromal level and strengthacross the multiple different layers of the dental prosthesis of FIG. 3g;

FIG. 3k is a schematic view showing the green body dental prosthesis ofFIG. 3b being colored;

FIGS. 4a-d are cross-sectional side schematic view of dental prosthesesof the present invention with the layers thereof having differentoptical properties;

FIGS. 5a and b are graphs of representative x-ray diffraction patternfor an exemplary zirconia disc of the present invention;

FIG. 6a is a schematic showing a qualitative translucency assessment ofthe current invention after final sintering;

FIG. 6b is a graph depicting the translucency of the current inventionat 600 nm for each level of differing material in the restoration afterthe final sintering stage;

FIG. 7a is a schematic diagram for measuring translucency of samplesusing a high-end spectrophotometer with an integrating sphere;

FIG. 7b is a graphical representation of light transmittance (%) versuswavelength (nm) at 400 to 800 nm for each level of differing material inthe restoration after final sintering; levels are labeled as samples A,B, C, D, and E of the current invention;

FIG. 8. is a schematic of the CIE L*a*b* colorimetric system to helpunderstand the color aspect of the current invention;

FIGS. 9a and 9b are schematic views of a color chroma measuring methodusing a hand-held spectrophotometer;

FIG. 10a is a schematic cross-sectional side view of a dental prosthesishaving different color chroma layers;

FIG. 10b is a graphical representation showing the color chroma levelfor samples A, B, C, D, and E of FIG. 10a ; and

FIGS. 11a and b are schematic views of coloring of a green body dentalprosthesis.

Reference will now be made to the exemplary embodiments illustrated, andspecific language will be used herein to describe the same. It willnevertheless be understood that no limitation of the scope of theinvention is thereby intended.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENT(S) Definitions

The terms “zirconia green body” and “green body” are usedinterchangeably herein to mean a three-dimensional granular structurecomprised of zirconia oxide particles, which is not sintered yet or,more frequently referred to, is partially sintered, pre-sintered or softsintered at a temperature of 900-1100° C., to facilitate millability ofthe disc/blank. The terms “green body dental prosthesis” and “greendental prosthesis” are used interchangeably herein to mean a dentalprosthesis that has been milled from the green body, but has not yetbeen sintered to become the final dental prosthesis.

The terms “pre-sintering” and “soft sintering” and “partial sintering”are used interchangeably herein to mean a reduction of size and/ornumber or the elimination of interparticle pores in a granular structurecomprised of particles by heating, without melting, of the particles.Pre-sintering is carried out at a temperature of around 900-1100° C. tofacilitate the machine milling of molded zirconia disc/blank. Afterpre-sintering zirconia is still porous and as a result becomes easy forcolor-ion liquid application. Pre-sintering or soft sintering isperformed on the cast or molded zirconia to obtain a green body withsufficient strength to be milled.

The terms “sintering” and “primary sintering” and “final sintering” areused interchangeably herein. After the green body of the specific dentalrestoration (or green body dental prosthesis or green dental prosthesis)is ready from the milling, primary/final sintering is done at a muchhigher temperature (around 1300-1600° C.) than pre-sintering. Afterprimary sintering, zirconia gets full densification, over 99%, andreaches its full flexural strength. Sintering is performed on the milled(and colored) green body dental prosthesis to obtain a dental prosthesiswith final strength and optical characteristics, such as translucencyand/or color intensity/chroma.

The term “zirconia” refers to various stoichiometries for zirconiumoxides, most typically ZrO2, and may also be known as zirconium oxide orzirconium dioxide. The zirconia may contain up to 20 weight percent ofoxides of other chemical elements such as, for example, oxides ofyttrium (e.g., Y₂O₃).

The term “ceramic” means an inorganic non-metallic material that isproduced by application of heat. Ceramics are usually hard, porous andbrittle and, in contrast to glasses or glass ceramics, display anessentially purely crystalline structure.

The term “glass ceramic” means an inorganic non-metallic material whereone or more crystalline phases are surrounded by a glassy phase.

The term “dental milling disc/blank” is a solid form of various shapes,e.g., disc or block or any shape that can be fixedly attached to thedental milling machine. Diameter for disc shape is usually 100-90 mm,with various thickness of 10-25 mm for multiple-prostheses milling.Blocks may be about 20 mm to about 30 mm in two dimensions (width andheight), for example, and may be of a certain length in a thirddimension.

The term “thickness” when used in reference to the green body, greenbody dental prosthesis, or the dental prosthesis refers to a particulardirection aligned in the thickness or height of the green body or dentalprosthesis, and can be from a lower layer or portion of the green bodyor dental prosthesis (corresponding to an cervical area of a tooth) toan upper layer or portion of the green body or dental prosthesis(corresponding to a incisal area of a tooth), such as an increasingtranslucency or decreasing chroma from the lower layer or portion(cervical) to the upper layer or portion (incisal).

DESCRIPTION

The current invention relates to a method of fabricating yttriastabilized polycrystalline zirconia discs/blanks to produce dentalprostheses using CAD/CAM processes. The blanks contain a graduallyincreasing amount of yttria (Y2O3) where the incisal area of a toothwill be, resulting in more translucency and less color intensity/chroma,thereby better replicating what is typically found in the human tooth.This inventive ceramic disc/blank does not have any optical gradationproperties in the green stage before primary sintering. Dentalprostheses made of this material take on similar optical propertiesfound in natural human teeth only after the coloring and sinteringstage.

Computer-aided design/computer-aided manufacturing (CAD/CAM) processesand equipment have been widely utilized in the dental industry. In theseprocesses a three-dimensional image of a stump of a tooth is createdalong with the teeth surrounding the stump in an effort to create adental restoration (dental prosthesis) which is to be placed over thestump. This image is displayed on a computer screen. Based on the stumpand surrounding teeth, the dental technician may then select a toothfrom a plurality of tooth library forms stored in the computer to bestfit the stump. The selected tooth is projected onto the stump until anoptimum positioning and fit of the dental restoration is achieved bydental design software. The digital data concerning the dentalrestoration thus formed are supplied to a numerically controlled millingmachine operating in three dimensions. The milling machine cuts a blankof ceramic material, typically zirconia, into the dental restorationdesign based on the data supplied.

Referring to FIG. 1, a method for fabricating a dental block is shown insteps 1-5; while a method for forming a dental prosthesis from thedental block is shown in steps 6-11. The starting zirconia material(3YS, 3YS-E, Px242, Tosoh Corp, Japan) consists of fairly uniformparticles thoroughly dispersed to be essentially free of agglomeratessuch that it will sinter predictably and isotropically withoutappreciable distortion. The particle size D50 may be in the range ofabout 0.1 to 1.0 micron. The zirconia and yttria can be formed into adesired shape (see 20, 24, 25 and 25 b in FIGS. 2a-2e by way ofexample), and the amount of ytrria can be increased through a thicknessof the shape or dental block. As show in table 1, zirconia material foreach different layer can be prepared by combining the zirconia andyttria together, while increasing the amount of yttria (Y2O3) insuccessive layers so that the amount of yttria is increasedincrementally from a lower layer to an upper layer. The amount of yttria(Y2O3) in in the lower layer can be 4.5-6 wt % in one aspect, or4.95-5.35 wt % in another aspect. The next upper layer has a higheramount of yttria (Y2O3) within the practical limit. The top layer canhave as high as 6.0-7.0 wt % of Y2O3, and it further can have as high as7.0-8.0 wt % of Y2O3, and yet it further can have as high as 8.0-9.0 wt% of Y2O3, and it can even have as high as 9.0-10 wt % of Y2O3.Incremental addition of yttria (Y2O3) can be done with theco-precipitation of Y2O3 with ZrO2 salts or by coating of the ZrO2grains with Y2O3.

TABLE 1 Color Zirconia Yttria (Y2O3) Pigment C ZrO2 + HfO2 + Y2O3 >99.00wt % 6.0-7.0 wt % 0.0 wt % B ZrO2 + HfO2 + Y2O3 >99.00 wt % 5.5-6.0 wt %0.0 wt % A ZrO2 + HfO2 + Y2O3 >99.00 wt % 4.9-5.35 wt %  0.0 wt %

Referring to FIGS. 2a-2d , zirconia powders and yttria are combined withor without a binder and pressed into blocks or similar shapes to form agreen body 20. Each layer or portions 21, 22, 23 of the green body 20 isdeposited using any of the known forming methods including, but notlimited to, pressing, uniaxial or isostatic, extrusion, slip casting,gel casting, pressure filtration and injection molding. In one aspect,the method to consolidate green body 20 is cold isostatic pressing (CIP)and pressure filtration which is associated with one of the highestdegrees of homogeneity attainable in green density. The zirconia andyttria of each layer can be separately combined and formed. Thus, thelayers have different chemical compositions, namely with differentamounts of yttria. The multiple different layers can be separate anddiscrete and distinct layers connected together but with a boundarytherebetween and characterized by distinct changes in the amount ofyttria; or the multiple different layers can form a region or layer witha more continuous change in the amount of yttria through the thicknessof the region or layer, with the multiple different layers beingindistinct and without any clear boundary therebetween. The height ofthe layers can be any proportion, and can be in a decreasing manner fromthe lower layer to the upper layer so that the lower layer is thickerand the upper layer is thinner (A>B>C). For example, bottom layer 21 canbe 2-5 mm thick, and the middle layer 22 can be 4 mm thick, and the toplayer can be 2 mm thick. The top layer 23 can be as thin as 2 mm, and itfurther can be as thin as 1 mm, and it further can be as thin as 0.5 mm.The layers can be as many as three layers, and it further can be as manyas up to seven to ten layers to create a natural transition of opticalchanges. The green body 20 is formed into any desired shape andconfiguration (such as a disc or puck 20, an elongated block or bar 24,or a square or rectangular block 25) which will render a dentalrestoration. The layers can be parallel and have a constant thickness,as shown in FIGS. 2a-2d . Alternatively, the layers can have anon-uniform thickness as shown in block 25 b in FIG. 2e . Fairlyuniform, free flowing particles should be used for pressing or molding.Binders such as polyvinyl alcohol (PVA), polyethylene glycol, wax, TEOS,and the like may be mixed with zirconia powders to retain the shape ofthe green bodies during and after forming. The invention is in no waylimited to the stated binders, and any suitable binder may be usedherein to achieve the desired results. The density of the green body 20is from about fifty percent (50%) to about seventy-five percent (75%)percent of theoretical density. In accordance with the process herein,after forming the zirconia ceramic powder into green body 20, the body20 may be machined to the shape of a dental restoration or green dentalprosthesis 26 such as a coping or full contour prosthesis, using acomputer assisted miller.

For purposes of example, a full contour zirconia tooth 26 will be usedto explain the process herein. The shape of the full contour zirconiatooth 26 is determined from data received by scanning the tooth or dieof the tooth to be restored. The size of the tooth which is machined isoversized to allow for shrinkage when the full contour zirconia tooth 26is sintered. The linear dimensions of the zirconia prosthesis 26 istypically about twenty percent (20%) to about twenty-five percent (25%)larger than the size of the final tooth since the linear shrinkage ofthe tooth after sintering is about sixteen percent(16.67%,=((1.20−1)/1.20) to about twenty percent (20%, =((1.25−1)/1.25).The full contour zirconia green body tooth 26 is then sintered to fulldensity at a temperature around 1300-1500° C. depending on the grainsize of the zirconia.

The green body 20 is soft-sintered to a bisque density that is betweenabout fifty percent (50%) and about eighty-five percent (75%) of thefinal density. The disc/blank 20 is treated with heat for millablestrength with temperature ranging from about 900 to about 1100° C. for aholding period of about 1 to 3 hours. A pre-sintered dental ceramicarticle or green dental prosthesis 26 typically has a density (usually3.0 g/cm 3 for an Yttrium stabilized ZrO 2 ceramic) that is lesscompared to a completely sintered dental ceramic article or dentalprosthesis 32 (usually 6.1 g/cm 3 for an Yttrium stabilized ZrO 2ceramic). After this pre-sintering stage, the current invention takes ona generally opaque appearance over the entire surface. The green body 20and/or 26, or layers thereof, can be substantially opaque with asubstantially consistent optical characteristic of non-translucency withrespect to visible light across the layers. (Various different dentalprostheses are shown in FIGS. 4a-4d with different numbers of layers anddifferent thicknesses of layers; and even different orientation oflayers; with the layers having different optical properties oftranslucency and/or chroma between adjacent layers.)

In the pre-sintering stage, the inventors found that the amount of openpores 27 between grains 28 (FIG. 3d ) are important because itdetermines the efficiency level of coloring at the later stage. The moreopen pores 27, the weaker the green body 26, but higher coloringefficiency, and the less the amount of open pores 27, the stronger thegreen body but lower coloring efficiency. The level of open pores 27that contain air can determine the desired green body strength formillability and the efficiency of green body 26 coloring. The level ofamount of open pores 27 can be expressed, for example, by L* value fromthe CIE L*a*b* colorimetric system.

The L*a*b* colorimetric system in FIG. 8 was standardized in 1976 byCommission Internationale de I'Eclairage (CIE). In the system, alightness/brightness is defined as L* and expressed by a numerical valueof from 0 to 100, in which L*=0 means that the color is complete black,and L*=100 means that the color is complete white.

When advanced ceramics with poly-crystal structures containsubstantially no residual pores after being fully sintered, the L* valuegoes up to as high as 60-85 for the samples with thickness of 1 mm,thereby characterized with good light transmission. When the zirconiagreen body 20 is partly sintered, i.e. in a pre-sintered stage, itcontains open pores/air which causes the diffusion of light, resultingin a much lower L* value number.

The inventors discovered that the higher this L* value number for thezirconia green body 20, the harder it is to mill and more difficult itis to be penetrated with color-ion liquids. The lower the number, theweaker the green body 20, causing cracks and chipping during the millingprocess and making it more difficult to control the coloring consistencyat later stage. It was found that the ideal L* value, when expressed inCIE L*a*b* colorimetric system in a standard illuminant D65, is between10 and 20 in one aspect, and 15 and 20 in another aspect.

Specifically, when measured for L* value of a CIE L*a*b* colorimetricsystem using the VITA Easyshade® Compact spectrophotometer (VITA,Germany, www.vita-zahnfabrik.com) 91 as in FIG. 9, which is most widelyused for color analysis in dental office/laboratory, the L* value of thecurrent invention, from a sample 29 with a diameter of 15 mm andthickness of 1.00 to 1.30 mm, is 10-20 in one aspect or 15-20 in anotheraspect from single and/or multi mode. The samples were measuredaccording to the user manual in such a way as for the reading tip 92 ofthe spectrophotometer 91 to be set flush with, in close touchingcontact, and perpendicular to the measured surface of the pre-sinteredzirconia sample 29. Since the VITA Easyshade has a built-in light sourceinside the tip area, the ideal L* value of 10-20 or 15-20 wereindependent of the amount of light in a normal office room setting.

The pre-sintered/soft-sintered disc/block 20, 24, 25 is then machinedwith a computer assisted miller 30 to a desired tooth shape 26, which isoversized to account for anticipated shrinkage during the sinteringstage, and sintered to a final density rendering a high strength dentalrestorative material or dental prosthesis 32. The soft-sintered state ofthe disc/blanks 20 allows for easy milling into complex or elaborateshapes 26.

Depending upon the density of the production batches, the lineardimensions of the green body prosthesis 26 may range from a size that isabout twenty percent (20%) to about twenty five percent (25%) largerthan the size of the final prosthesis 32 based on the linear shrinkageof the bisque body 26 which may range from about sixteen percent(16.67%, =((1.20−1)/1.20) to about twenty percent (20%, =((1.25−1)/1.25)shrinkage thereof. The green body prosthesis 26 is then sintered to fulldensity at the temperature-time cycle specific for the zirconia grainsize used, e.g., for 0.1-1.0 micron, at about 1300-1600° C. for about 1to 3 hours.

Unlike the glass and glass-ceramic materials, coloring of the currentinvention is done after pre-sintering, that is, in a porous andpartially sintered state. For all the glass ceramic materials, colorpigments are added to the glass matrix and then glass ceramic undergoesheat treatments. The coloring of the current invention involves the useof aqueous color solution. This coloring of zirconia 26 is processed inthe porous or absorbent state, which is characterized in that metal ionsolutions or metal complex solutions are used for the coloring. Aqueousor alcoholic metal solutions of Fe, Mn, Cr can be used, for example aschlorides or acetates. Milled green prostheses 26 are dipped into thesolution or can be brushed for specific tooth shade effects. Thiszirconia green body 20 and milled chalky parts 26 are not a ceramiccompound with predetermined optical properties, instead they are aceramic compound without predetermined optical properties. They arechalky, opaque, do not have translucency as shown in FIG. 3e , and donot have difference in brightness, reflectance or color between theupper and lower portions of the disc/blank as shown in FIG. 3 f.

Pre-sintered and milled zirconia prosthesis 26 shrinks during a(primary) sintering step, that is, if an adequate temperature isapplied. The sintering temperature to be applied depends on the ceramicmaterial chosen. For ZrO 2 based ceramics a typical sinteringtemperature range is about 1300° C. to about 1600° C. Al 2 O 3 basedceramics are typically sintered in a temperature range of about 1300° C.to about 1700° C. Glass ceramic materials are typically sintered in arange of about 700 to about 1100° C. for about 1 to about 3 h. Thisprimary sintering includes the densification of a porous material to aless porous material (or a material having less cells) having a higherdensity, and in some cases sintering may also include changes of thematerial phase composition (for example, a partial conversion of anamorphous phase toward a crystalline phase).

Pre-sintered, yttrium-stabilized zirconia ceramics 20 are available foruse with CAD/CAM technologies. Zirconia ceramic can be used forframeworks of crowns and FDP (fixed dental prosthesis) in the posteriorregion. Unfortunately, current processing technologies cannot makezirconia frameworks or full contour crowns as translucent as naturalteeth.

Translucency is the relative amount of light transmitted through thematerial. In a natural tooth, translucency is identified when anoticeable amount of light passes through its proximal and/or incisalaspect due to the presence of only enamel or a high proportion of enamelcompared to the underlying dentin. In the cervical aspect of the teeth,where the dentin is thicker, the light transmission will be reduced. Thetranslucency of the enamel and dentin is wavelength dependent; thehigher the wavelength, the higher the translucency value.

The human tooth structure scatters much of the incidental light. In suchlight scattering media, the intensity of the incidence light flux isdiminished as the light passes through the medium. The enamel and dentinare not totally homogeneous at the histological level, which affectsscattering and absorption of the light.

One method of measuring translucency is by determining totaltransmission, including scattering, using a spectrophotometer with anintegrating sphere as shown in FIG. 7a . Translucency of a material canbe expressed as a transmission coefficient or total (direct anddiffused) light transmittance (%) as the relative amount of lightpassing through the unit thickness of the material. To measure thedifferent level of translucency of each layer of the current invention,total light transmittance was measured by a double beam-systemspectrophotometer 71 as in FIG. 7a (LAMBDA 35, UV/Vis Spectrophotometersmanufactured by Perkin Elmer, USA, with a visible light lamp rated atapproximately 3100 Kelvin) based on the “Standard test method fortransmittance and color by spectrophotometer using hemisphericalgeometry” of ASTM E1348-11 and “Materials and articles in contact withfoodstuffs—Test methods for translucency of ceramic articles” of DanskStandard/EN 1184.

Measurement samples 61-64 used in FIG. 6a were samples obtained byprocessing a fully sintered (holding time 2 hours at 1,530° C. withregular sintering in the air, no post HIP processing) body of thecurrent zirconia invention to a thickness of 0.6 mm (with a diameter of20 mm) and mirror polishing both sides with 2500-grit silicon carbidepaper (Wetordry Tri-M-ite Paper, 2500A, 3M Company, St. Paul, Minn.).All the samples were cleaned with isopropyl alcohol before lighttransmittance measurement. Light emitted from a light source (deuteriumlamp and halogen lamp) was passed through a sample and scattered, andall light transmission amount was measured using an integrating sphere.Samples of small plates a, b, c and d from each layer of the currentinvention 20 as shown in FIG. 6a have incrementally increasing amountsof yttria (Y2O3) as a component. Each material constituting each layerB, C, D and E was used to make small plates b 61, c 62, d 63 and e 64 of0.6 mm in thickness. As yttria (Y2O3) contents increase, so does totallight transmittance as shown in FIG. 6 b.

Visible light which had passed through the sample was collected with anintegrating sphere to determine the intensity of the visible light (I).On the other hand, the intensity of visible light (I.sub.0) was measuredwithout placing the sample. The total light transmittance was calculatedin terms of the proportion of the former to the latter intensity(=I/I.sub.0). A transmission spectrum and digital data record wereobtained for each measurement with the light beam entering the samples.Four measurements were made with each sample rotated 90° from theprevious measurement.

A measurement wavelength region was from 400 to 800 nm, and total lighttransmittance in the present invention was a transmittance at awavelength of 600 nm in a visible light region as shown in table 2 andFIG. 7b . The decrease in total light transmittance with decreasingwavelength is due to the increase in light scattering as indicated bythe Rayleigh scattering equation. Similar results were reported fordental porcelains.

Test result of all light transmittance is presented in the followingcomparative example.

Comparative Example

TABLE 2 Stabilizer (Y2O3) sintering Density of Flexural total lightSamples of contents temperature sintered body strength* transmittance0.6 mm thick Wt % ° C. g/cm3 Mpa % Sample E 7.0 1530 99.8 925 52.91 (5thlayer) Sample D 6.5 1530 99.8 1021 51.55 (4th layer) Sample C 6.0 153099.8 1152 51.08 (3rd layer) Sample B 5.5 1530 99.8 1253 50.92 (2ndlayer) Sample A 4.9-5.35 1530 99.8 1385 49.72 (1st layer)

The inventors discovered that the level of total light transmittance (%)increased when yttria contents were increased towards the next upperlayers. As can be seen from the table 2, each upper layer has a higherlight transmittance than all layers below it. All of the 0.6 mm thicksamples have a higher transmittance level that is approximately 12-16%higher than 1.0 mm thick samples tested with the same procedures andmethods.

The bottom layer (first layer) 21 of the current invention disc/blank inwhich cervical aspect of dental crown & bridge would be located has thelowest translucency level after primary sintering, representing thethinnest enamel and thickest dentin portion of a human tooth. Afterprimary sintering, translucency levels gradually increase towards thetop portion 23 of the current invention dental disc/blank. The topportion 23 has the highest level of translucency after primary sinteringin which the incisal aspect of dental prosthesis would be located.

The current invention is technically distinctive and differentiated fromother multi-layered ceramics in the way the graded translucency level iscreated. One type of multi-layered ceramic currently available in themarket comprises small sized rectangular blocks with pre-determinedcolors, for example, A1, A2, etc, that all have chemically homogenouscomponents for each layer in which, less colorants/color pigments areused towards the incisal area to produce a seemingly more translucenteffect. The yttria (Y2O3) of the present invention is not a colorant, itdoes not produce any color effect. The other material type is known thathas a different translucency level from the bottom (cervical) to the top(incisal).

Whereas, the green body dental block of the current invention does nothave any noticeable optical characteristics, except that it is generallyan opaque disk/blank without pre-determined color. Even if there aremore yttria (Y2O3) contents towards the incisal area of the currentinvention, the translucency level before primary sintering is still thesame for each layer, being only very opaque and substantially almost notranslucency throughout the whole green body 20, 26 as shown in FIG. 3e. The increased translucency effect can take place only after theprimary sintering stage is complete as in dental prosthesis 32.

Prior art teaches that the reason why each layer should not have adifferent chemical composition, but should only have a variation ofcontents of specific color pigments, is because of the coefficiency ofthermal expansion. The dental industry is characterized by accurate fitof the restorations with no distortion of the sintered prosthesis. Allof the dental ceramic materials have their unique pattern of response toheat when treated with high temperature for strengthening. The way eachmaterial behaves are all different, including but not limited to, thespeed by which porous ceramic material shrinks and the absolutetemperature level that requires full densification, etc. If the chemicalcomposition of each layer is different thereby creating a different CTE(Coefficiency of Thermal Expansion), sintering temperature, or sinteringspeed, then the final restoration would not fit in the patient's mouth.This is why all prior art of layered ceramics use the same chemicalcomposition throughout the blank, and put only different amounts oftooth color pigments from the bottom portion to the top portion.

The current invention was able to overcome this area of CTE relatedproblems inspite of having different chemical compositions withdifferent levels of yttria (Y2O3) contents by individuallycharacterizing, for example, coating powder particles of each layer.

Yttria (Y2O3) is just an example of some of the additives/componentsthat increase the translucency of the incisal/top area of dental blanks.Other examples that produces the similar effect are spinel (MgAl2O4),Al2O3, SiO2, TiO2, B203, Na2O3, Y2O3, K2O, CeO2, MgAl2O4, MgO, HfO2,etc.

Natural teeth are typically composed of a variety of colors, and agradation occurs in an individual tooth from the gingival margin to theincisal edge depending upon the ratio between enamel and dentinthickness.

One of the most exacting and time consuming aspects of dentalrestorations, whether involving direct or indirect placement techniques,is that of properly matching the color of the restoration to that of theoriginal tooth. In the context of clinical dentistry, the term “color”involves three discrete concepts: hue, chroma and value. Hue is thedimension of color that enables us to distinguish one family of colorfrom another; chroma defines the relative intensity/saturation of aparticular color, i.e., the more intense a color is, the higher itschroma level; and value describes the relative whiteness or blackness ofa particular color, i.e., the brighter the color, the higher its value.

Color is often defined in terms of its CIELAB lightness value, L*, itsCIELAB chroma value, C* and its CIELAB hue value, h. “CIE” stands forthe Commission Internationale de l'Eclairage and its CIELAB L*, C* and hvalues are well known and widely used. “Lightness”, L* value, is ameasure of the amount of light/brightness reflected from a surface, thatis, the amount of white or black in a color, its lightness or darkness.“Chroma”, C*, is a measure of the intensity of a color, ie. the extentto which it is either a pastel color or a strong color or something inbetween. “Hue”, h, is a measure of how reddish, yellowish, greenish orbluish a color is.

Color matching in dentistry is routinely performed with a visual method.However, instrumental color measurement can render useful informationthat can aid visual color matching. The Commission Internationale del'Eclairage (CIE) refined color space in 1976 as shown in FIG. 8. CIE L*value is a measure of the lightness of an object such that a perfectblack has a CIE L* value of zero and a perfect reflecting diffuser(white) has a CIE L* value of 100. CIE a* value is a measure of redness(positive value) or greenness (negative value), and CIE b* value is ameasure of yellowness (positive value) or blueness (negative value). Asshown in FIG. 8, The black vertical line is the L* value intensity axis,the hue is given by an angle from the L* value intensity axis and thechroma/saturation is the distance from the L* value intensity axis tothe color point (i.e., the radius). The larger the numerical value ofeach of a* and b* is, the brighter the color becomes, whereas when thesmaller the numerical value of each of a* and b*, the duller the colorbecomes.

Spectrophotometric color measurements differ depending on the measuringgeometry and the illuminant. Therefore, when any color measurements aremade with an instrument, measured color values are sensitive to themethods employed. Several standard illuminants have been used to measurethe color of dental materials. Standard illuminant D65 represents aphase of daylight with a correlated color temperature of approximately6500 K, illuminant A represents light from the full radiator at absolutetemperature 2856 K, and illuminant F2 represents light from fluorescentlamp of medium color temperature of 4230 K. Two standard illuminants arerecommended for use in colorimetry. Illuminant A should be used in allapplications of colorimetry involving the incandescent lighting, and D65should be used in all colorimetric calculations requiring representativeday light.

To standardize the light source mentioned above and easily calculate thechroma level VITA Easyshade® Compact spectrophotometer (VITA, Germany,www.vita-zahnfabrik.com) was used as in FIG. 9.

Chroma level was calculated as C*ab=(a*2+b*2)½ according to“Colorimetry-technical report. CIE Pub. No. 15, 3rd ed. Vienna: BureauCentral de la CIE; 2004”

The inventors found out that an increased amount of Y2O3 within apractical limit (0.1-3.0 wt %) for each layer as an additive in thezirconia (ZrO2) body produces a sintered body that has lower intensityin chroma after being dipped into the color-ion solution for shadingeffect. The more Y2O3 is used within the practical limit, the sinteredbody becomes lighter in color intensity/chroma.

For example, as seen in the following table 3, the first layer 101 inwhich the cervical aspect of a tooth will be located has the strongestcolor intensity/chroma and the fifth layer 105 in which the incisalportion of a tooth would be located has the weakest colorintensity/chroma. When dipped into a specific color liquid 31, sample a(first layer 101) produced a slightly dark redish brown color afterprimary sintering, whereas the sample c (third layer 103) produced amoderately redish brown color, and e (fifth layer 105) produced asintered body with a light ivory brown color. When each of the differentlayers of zirconia body 21, 22, 23, with an increasing amount of Y2O3,was deposited into one body 20, 26, 29 and dipped into a specific color(hue) liquid 31, the subsequently sintered body 32, 33 showed a gradedcolor intensity (chroma). This means that a layered zirconia body 20,26, 29 after being dipped into a color liquid 31 would be able toproduce a subsequent sintered body 32, 33 that is gradually diminishingin color intensity/chroma from the cervical to incisal direction as istypically found in the human tooth. Color liquids 31 can be pre-made inas many colors as needed, and the milled porous prosthesis 26 can besimply dipped into this liquid 31. In this way the current invention 20makes it unnecessary to keep all the inventories of blocks/discs 20 ofdifferent shades. Thus, the green body dental prosthesis can be dippedinto a single color liquid.

TABLE 3 Y2O3 Samples (thickness 1 mm) (Wt %) CIE a* CIE b* CIE c*abSample E (fifth layer E) 6.50 −2.4 10.2 10.4 Sample D (forth layer D)6.25 −1.9 13.5 12.5 Sample C (third layer C) 6.00 −1.2 16.4 16.4 SampleB (second layer B) 5.50 −0.2 22.2 22.2 Sample A (first layer A) 5.00 2.028.8 28.9

The chroma was measured utilixing a VITA spectrophotometer 91 Easy Shadeaccording to the user manual. The tip 92 of the device 91 was flush andperpendicular with sample 29 as shown in FIG. 9. The result wasindependent of lighting condition in the office room, that is, themeasurement reading did not change when measured with or without theindoor (fluorescent) light on.

This feature of a gradual decrease in color intensity/chroma with highertranslucency towards the incisal area gives this invention a uniquebenefit and advantage of being very similar in optical properties to anatural human tooth and is therefore distintly set apart over othermonolithic zirconia bodies currently available in the market. Coloringof dental zirconia bodies, until now, has been possible with only twomethods. One is using a non-colored zirconia body that is (subsequent tomilling but before final sintering) treated with color-ion liquid. Thebenefit of this method is to be able to avoid the need of keeping alarge inventory of different colored blocks/discs. The disadvantage isthat the color of the sintered zirconia body is only mono-chromaticsince the color-ion responds the same all the way throughout thehomogeneous component of the discs/blocks and finally to the dentalprosthesis, making a restoration with only one color. The other methodis to use a pre-colored blocks in which each layer has already beenpre-colored with different levels of color pigments, but the primarycomponents are basically the same. The advantage is to avoid thecoloring process, but the disadvantage is the inefficiency associatedwith large inventories and restrictions on milling many differentcolored prostheses in one milling sequence. The current invention hasthe benefits of both methods by the fact that 1) it uses thesimple-dipping coloring method which eliminates the inefficienciesassociated with inventory issues and 2) sintered results give the gradedcolor intensity/chroma like the pre-colored blocks with different levelsof color pigments. This unique feature comes from a disc/blank in whicheach layer has been prepared hetero-geneously and deposited withdifferent levels of yrrtia (Y2O3) contents.

The current invention of layered zirconia body 20 also shows afunctionally graded flexural strength. As shown in table 2, the firstlayer a 101 with the lowest amount (wt %) of yttria (Y2O3) contentsshows the highest strength of 1385 MPa, and the fifth layer e 105 withthe highest amount of (wt %) of yttria (Y2O3) contents shows the loweststrength of around 925 Mpa. The flexural strength was based on “Implantsfor surgery-ceramic materials based on yttria-stabilized tetragonalzirconia” of ISO 13356, and the flexural strength was measured by athree-point flexural test.

It is already know in the industry that color reproduction of dentalprosthesis can be done using the method from U.S. Pat. No. 6,709,694. Asshown in FIGS. 11a, and 11b , milled porous dental prosthesis 26, beforeprimary sintering, can be completely immersed in a color-ion liquid 31for a specific tooth color and then, after a drying time of about 30minutes, be sintered at temperature of around 1,500° C. in the sinteringfurnace to produce a color effect of the tooth. The problem with theabove mentioned method is that the immersed dental prosthesis body 26absorbs the coloring liquid 31 in a homogeneous way throughout theentire prosthesis 26, resulting in a sintered prosthesis that has allthe same color from the bottom/cervical to the top/incisal direction.It's either in all incisal-light color or all body-intense color. Onthis starting prosthesis 26, which has only a mono-tone color throughoutthe sintered body, the dental technician has to add extra colors tocreate a darker effect for the body and cervical aspect of a tooth. Theteaching of this method is a two-step coloring by which an overall lightcolor has to be first created corresponding to the incisal color of atooth and then a darker body color is created at a separate, subsequentheat treatment process.

What the inventors discovered is a simplified one-step sintering method,while still allowing the creation of a double coloring effect asindicated in the following method; first the liquid 110 is applied onthe incisal area only with a brushing method, followed by a smearingtime of about 30 seconds, and second the liquid 31 is applied by theimmersing method. The components of the first liquid 110 that is brushedon the incisal area are water (40-45 wt %), polyethylene glycol (40-45wt %) and manganese chloride for av incisal graying effect.Coloring-ions other than colorants for graying effect preferably are notadded. The inventors found that polyethylene glycol in the first liquid110 plays a role of either partly blocking or interfering with theinfiltration of the second color liquid 31 into the area applied by thefirst liquid 110, which is a very useful discovery that has not beentaught anywhere. The principle of coloring with color liquids 110 and 31is that the color ion in the liquid 110 smears through the porous space(about 50-100 nano) of the pre-sintered green zirconia body 26 describedin the earlier part of the detailed description of this invention. Butwhen this porous space is already filled with liquid agents likepolyethylene glycol, the infiltration process becomes locally deterredand incomplete. As a result, the incisal area covered by the firstliquid 110 becomes light in color intensity/chroma presenting morenatural tooth color characteristics.

This method can be used for both monolithic zirconia that is all onematerial/component or multi-layered zirconia bodies with differentamounts of yttria (Y2O3) contents. When used with multi-layered zirconiabodies, the result is more aesthetic, since the incisal aspect of themulti-layered zirconia, with increasing amounts of yttria (Y2O3), givesincrementally increased translucency as well.

There should be a certain period of absorbing and drying time of thefirst liquid 110 of about 0.5 to 2 minutes for optimum results beforethe application of the second liquid 31. The first liquid can be locallyand incisally applied by using a fine tipped brush 111, and theapplication of the second liquid 31 is usually completed by immersingthe green body 26 in the color liquid 31. Moving brush 111 from theinsical to body direction makes it possible to apply more of the firstliquid in the top portion of incisal and less of the first liquid in thelower portion of the incisal area, allowing a smooth color transitionfrom the incisal area to the body area of a tooth. After being removedfrom the second liquid 31, the prosthesis 26 is dried under a light andsintered in a regular way. Then the sintered body displays an idealmultiple color intensity effect with less color/chroma (with optionalincisal gray effect with manganese chloride) in the incisal area andmore color in the body and cervical area. There is a gradual transitionarea between incisal and body of a sintered prosthesis. Theeffectiveness of this coloring method of creating gradually decreasingcolor intensity/chroma towards the incisal area of a dental prosthesiscan be increased when used with a multi-layer zirconia body that hasincreasing translucency towards the incisal area of a dental prosthesis.

In accordance with another aspect of the present invention, a largersize of blank (typically a round disc) of non pre-colored ceramicmaterial can be more efficient in that the operator can mill multipleteeth all at the same time from the same un-colored discsregardless ofindividual characteristic requirements and do the coloring job at aseparate stage later.

While the forgoing examples are illustrative of the principles of thepresent invention in one or more particular applications, it will beapparent to those of ordinary skill in the art that numerousmodifications in form, usage and details of implementation can be madewithout the exercise of inventive faculty, and without departing fromthe principles and concepts of the invention. Accordingly, it is notintended that the invention be limited, except as by the claims setforth below.

What is claimed is:
 1. A method of making a translucent colored zirconiadental restoration, the method comprising the steps of: a) obtaining azirconia green body comprising: i) zirconium oxide and between 7 wt % to20 wt % of another oxide based on a total weight percent of the zirconiagreen body; ii) an L* value between 10 and 20 for a sample thickness of1 to 1.3 mm in accordance with CIE L*a*b* colorimetric system, measuredwith a reading tip of a spectrophotometer flush with, in close touchingcontact, and perpendicular to a measured surface of the sample; and iii)the zirconia green body being subsequently finally sinterable withregular sintering in air without post HIP processing to produce atranslucent zirconia sintered body having a total light transmittance ofat least 36% and less than 50% to light with a wavelength of 400 nm, andless than 55% to light with a wavelength of 600 nm, at a thickness of0.6 mm measured using a LAMBDA 35 UV/VIS Spectrophotometer manufacturedby Perkin Elmer; and then b) forming a dental restoration precursor fromthe zirconia green body; and then c) applying a color liquid to theprecursor; and then d) sintering the restoration precursor with regularsintering in air without post HIP processing resulting in thetranslucent colored zirconia dental restoration.
 2. The method inaccordance with claim 1, wherein the another oxide further comprises atleast one of MgAl₂O₄, Al₂O₃, SiO₂, TiO₂, B₂O₃, Na₂O₃, Y₂O₃, K₂O, CeO₂,MgAl₂O₄, MgO, or HfO₂.
 3. The method in accordance with claim 1, whereinforming the dental restoration precursor further comprises machining theprecursor from the zirconia green body.
 4. The method in accordance withclaim 3, wherein machining the restoration precursor further comprisesmachining the restoration precursor in an oversized manner to allow forshrinkage of the material during sintering.
 5. The method in accordancewith claim 1, wherein sintering the restoration precursor furthercomprises sintering at a temperature of 1,300° C. to 1,600° C.
 6. Themethod in accordance with claim 1, wherein the translucent coloredzirconia dental restoration comprises a crown, a bridge, an inlay, anonlay, an orthodontic appliance, a denture, a veneer, an implant or anabutment.
 7. The method in accordance with claim 1, wherein the totallight transmittance of the fully sintered zirconia restoration ismeasured with a light source that emits visible light from a halogenlamp.
 8. The method in accordance with claim 1, wherein the zirconiagreen body further comprises particles having a size D50 of about 100nanometers.
 9. The zirconia green body in accordance with claim 1,wherein the particles of the zirconia green body have a size D50 in therange of about 100 to 1000 nanometers.
 10. A method of making atranslucent colored zirconia dental restoration, comprising the stepsof: a) obtaining a zirconia green body comprising: i) zirconium oxideand between 6.5 to 20 wt % of another oxide based on a total weightpercent of the zirconia green body; ii) an L* value between 10 and 20for a sample thickness of 1 to 1.3 mm in accordance with CIE L*a*b*colorimetric system, measured with a reading tip of a spectrophotometerflush with, in close touching contact, and perpendicular to a measuredsurface of the sample; and iii) the zirconia green body beingsubsequently finally sinterable with regular sintering in air withoutpost HIP processing to produce a translucent zirconia sintered bodyhaving a total light transmittance of at least 35% and less than 50% tolight with a wavelength of 400 nm, and at least 48% to light with awavelength of 500 nm, and less than 55% to light with a wavelength of600 nm, at a thickness of 0.6 mm, measured using a LAMBDA 35 UV/VISSpectrophotometer manufactured by Perkin Elmer; and then b) forming adental restoration precursor from the zirconia green body, and applyinga color liquid to the precursor; and then c) sintering the restorationprecursor with regular sintering in air without post HIP processingresulting in the translucent colored zirconia dental restoration. 11.The method in accordance with claim 10, wherein the another oxidefurther comprises at least one of MgAl₂O₄, Al₂O₃, SiO₂, TiO₂, B₂O₃,Na₂O₃, Y₂O₃, K₂O, CeO₂, MgAl₂O₄, MgO, or HfO₂.
 12. The method inaccordance with claim 10, wherein a light source to measure the totallight transmittance of the translucent zirconia sintered body emitsvisible light from a halogen lamp.
 13. The method in accordance withclaim 10, wherein the translucent colored zirconia dental restorationcomprises a crown, a bridge, an inlay, an onlay, an orthodonticappliance, a denture, a veneer, an implant or an abutment.
 14. Themethod in accordance with claim 10, wherein the zirconia green bodyfurther comprises: greater than 99 wt % of zirconia, yttria, and hafniabased on the total weight percent of the zirconia green body; and anamount of the yttria between 6.5 wt % to 20 wt % based on the totalweight percent of the zirconia green body.
 15. A method of making atranslucent colored zirconia dental restoration, comprising the stepsof: a) obtaining a zirconia green body with an L* value between 10 and20 for a sample thickness of 1 to 1.3 mm in accordance with CIE L*a*b*colorimetric system, measured with a reading tip of a spectrophotometerflush with, in close touching contact, and perpendicular to a measuredsurface of the sample, and the zirconia green body being subsequentlyfinally sinterable with regular sintering in air without post HIPprocessing to produce a translucent zirconia sintered body having atotal light transmittance of greater than 37% and less than 50% to lightwith a wavelength of 400 nm, and greater than 47% to light with awavelength of 500 nm, and less than 55% to light with a wavelength of600 nm, at a thickness of 0.6 mm, measured using a LAMBDA 35 UV/VISSpectrophotometer manufactured by Perkin Elmer; and then b) forming adental restoration precursor from the zirconia green body, and applyinga color liquid to the precursor; and then c) sintering the restorationprecursor with regular sintering in air without post HIP processingresulting in the translucent colored zirconia dental restoration. 16.The method in accordance with claim 15, wherein the zirconia green bodycomprises zirconium oxide and another oxide comprising at least one ofMgAl₂O₄, Al₂O₃, SiO₂, TiO₂, B₂O₃, Na₂O₃, Y₂O₃, K₂O, CeO₂, MgAl₂O₄, MgO,or HfO₂.
 17. The method in accordance with claim 15, wherein forming thedental restoration precursor further comprises machining the dentalrestoration precursor in an oversized manner to allow for shrinkage ofthe material.
 18. The method in accordance with claim 15, whereinsintering the restoration precursor further comprises sintering at atemperature of 1,300° C. to 1,600° C.
 19. The method in accordance withclaim 15, wherein a light source to measure the total lighttransmittance of the translucent zirconia sintered body emits visiblelight from a halogen lamp.
 20. The method in accordance with claim 15,wherein the translucent colored zirconia dental restoration comprises acrown, a bridge, an inlay, an onlay, an orthodontic appliance, adenture, a veneer, an implant or an abutment.
 21. A method of making atranslucent colored zirconia dental restoration, comprising the stepsof: a) obtaining a zirconia green body comprising: i) zirconium oxideand greater than 6.5 wt % and less than 20 wt % of another oxide basedon a total weight percent of the zirconia green body; ii) an L* valuebetween 10 and 20 for a sample thickness of 1 to 1.3 mm in accordancewith CIE L*a*b* colorimetric system, measured with a reading tip of aspectrophotometer flush with, in close touching contact, andperpendicular to a measured surface of the sample; iii) a chemicalcomposition with color pigments; and vi) the zirconia green body beingsubsequently finally sinterable at a temperature of at least 1300° C.with regular sintering in air without post HIP processing to produce atranslucent zirconia sintered body having a total light transmittance ofless than 50% to light with a wavelength of 400 nm, and less than 55% tolight with a wavelength of 600 nm, and at least 43% to light with awavelength at a point between 400 nm and 600 nm, and at least 50% tolight with a wavelength at a point between 600 nm and 800 nm, at athickness of 0.6 mm measured using a LAMBDA 35 UV/VIS Spectrophotometermanufactured by Perkin Elmer; and then b) forming a dental restorationprecursor from the zirconia green body; and then c) sintering therestoration precursor with regular sintering in air without post HIPprocessing resulting in the translucent colored zirconia dentalrestoration.
 22. The method in accordance with claim 21, wherein theanother oxide further comprises at least one of MgAl₂O₄, Al₂O₃, SiO₂,TiO₂, B₂O₃, Na₂O₃, Y₂O₃, K₂O, CeO₂, MgAl₂O₄, MgO, or HfO₂.
 23. Themethod in accordance with claim 21, wherein forming the dentalrestoration precursor further comprises machining the precursor from thezirconia green body.
 24. The method in accordance with claim 23, whereinmachining the precursor further comprises machining the restorationprecursor in an oversized manner to allow for shrinkage of the material.25. The method in accordance with claim 21, wherein a light source tomeasure the total light transmittance of the translucent zirconiasintered body emits visible light from a halogen lamp.
 26. The method inaccordance with claim 21, wherein the translucent colored zirconiadental restoration comprises a crown, a bridge, an inlay, an onlay, anorthodontic appliance, a denture, a veneer, an implant or an abutment.27. The method in accordance with claim 21, wherein the zirconia greenbody is subsequently finally sinterable to produce a translucentzirconia sintered body with multiple areas with each area havingdifferent chroma level (CIE C*ab) between adjacent areas.
 28. The methodin accordance with claim 27, wherein the chroma level is measured with areading tip of a spectrophotometer flush with, in close touchingcontact, and perpendicular to a measured surface of the translucentzirconia sintered body.
 29. The method in accordance with claim 21,wherein the zirconia green body further comprises: multiple differentareas each having a different chemical composition between adjacentareas; the different chemical composition including different amounts ofthe another oxide between the adjacent areas; and the amount of theanother oxide increasing from a lower area to an upper area.
 30. Themethod in accordance with claim 29, wherein the another oxide furthercomprises at least one of MgAl₂O₄, Al₂O₃, SiO₂, TiO₂, B₂O₃, Na₂O₃, Y₂O₃,K₂O, CeO₂, MgAl₂O₄, MgO, or HfO₂.